Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 80F85C61DA4 for ; Fri, 24 Feb 2023 15:30:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229840AbjBXPaw (ORCPT ); Fri, 24 Feb 2023 10:30:52 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55132 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229670AbjBXPav (ORCPT ); Fri, 24 Feb 2023 10:30:51 -0500 Received: from netrider.rowland.org (netrider.rowland.org [192.131.102.5]) by lindbergh.monkeyblade.net (Postfix) with SMTP id 4BD1D1DB8C for ; Fri, 24 Feb 2023 07:30:49 -0800 (PST) Received: (qmail 40397 invoked by uid 1000); 24 Feb 2023 10:30:48 -0500 Date: Fri, 24 Feb 2023 10:30:48 -0500 From: Alan Stern To: "Paul E. McKenney" Cc: Akira Yokosawa , Joel Fernandes , Andrea Parri , Boqun Feng , Daniel Lustig , David Howells , Jade Alglave , Jonas Oberhauser , Luc Maranget , Nicholas Piggin , Paul =?iso-8859-1?Q?Heidekr=FCger?= , Peter Zijlstra , Will Deacon , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v2] tools/memory-model: Add documentation about SRCU read-side critical sections Message-ID: References: <20230213015506.778246-1-joel@joelfernandes.org> <20230222195051.GT2948950@paulmck-ThinkPad-P17-Gen-1> <73baee18-802e-f694-a3d7-9ae776e6fd69@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <73baee18-802e-f694-a3d7-9ae776e6fd69@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Expand the discussion of SRCU and its read-side critical sections in the Linux Kernel Memory Model documentation file explanation.txt. The new material discusses recent changes to the memory model made in commit 6cd244c87428 ("tools/memory-model: Provide exact SRCU semantics"). Signed-off-by: Alan Stern Co-developed-by: Joel Fernandes (Google) Signed-off-by: Joel Fernandes (Google) Reviewed-by: Akira Yokosawa Cc: Andrea Parri Cc: Boqun Feng Cc: Jade Alglave Cc: Jonas Oberhauser Cc: Luc Maranget Cc: "Paul E. McKenney" Cc: Peter Zijlstra CC: Will Deacon --- v2: Add tags from Joel Fernandes and Akira Yokosawa. Correct a typo in the text (Akira). tools/memory-model/Documentation/explanation.txt | 178 +++++++++++++++++++++-- 1 file changed, 167 insertions(+), 11 deletions(-) Index: usb-devel/tools/memory-model/Documentation/explanation.txt =================================================================== --- usb-devel.orig/tools/memory-model/Documentation/explanation.txt +++ usb-devel/tools/memory-model/Documentation/explanation.txt @@ -28,9 +28,10 @@ Explanation of the Linux-Kernel Memory C 20. THE HAPPENS-BEFORE RELATION: hb 21. THE PROPAGATES-BEFORE RELATION: pb 22. RCU RELATIONS: rcu-link, rcu-gp, rcu-rscsi, rcu-order, rcu-fence, and rb - 23. LOCKING - 24. PLAIN ACCESSES AND DATA RACES - 25. ODDS AND ENDS + 23. SRCU READ-SIDE CRITICAL SECTIONS + 24. LOCKING + 25. PLAIN ACCESSES AND DATA RACES + 26. ODDS AND ENDS @@ -1848,14 +1849,169 @@ section in P0 both starts before P1's gr before it does, and the critical section in P2 both starts after P1's grace period does and ends after it does. -Addendum: The LKMM now supports SRCU (Sleepable Read-Copy-Update) in -addition to normal RCU. The ideas involved are much the same as -above, with new relations srcu-gp and srcu-rscsi added to represent -SRCU grace periods and read-side critical sections. There is a -restriction on the srcu-gp and srcu-rscsi links that can appear in an -rcu-order sequence (the srcu-rscsi links must be paired with srcu-gp -links having the same SRCU domain with proper nesting); the details -are relatively unimportant. +The LKMM supports SRCU (Sleepable Read-Copy-Update) in addition to +normal RCU. The ideas involved are much the same as above, with new +relations srcu-gp and srcu-rscsi added to represent SRCU grace periods +and read-side critical sections. However, there are some significant +differences between RCU read-side critical sections and their SRCU +counterparts, as described in the next section. + + +SRCU READ-SIDE CRITICAL SECTIONS +-------------------------------- + +The LKMM uses the srcu-rscsi relation to model SRCU read-side critical +sections. They differ from RCU read-side critical sections in the +following respects: + +1. Unlike the analogous RCU primitives, synchronize_srcu(), + srcu_read_lock(), and srcu_read_unlock() take a pointer to a + struct srcu_struct as an argument. This structure is called + an SRCU domain, and calls linked by srcu-rscsi must have the + same domain. Read-side critical sections and grace periods + associated with different domains are independent of one + another; the SRCU version of the RCU Guarantee applies only + to pairs of critical sections and grace periods having the + same domain. + +2. srcu_read_lock() returns a value, called the index, which must + be passed to the matching srcu_read_unlock() call. Unlike + rcu_read_lock() and rcu_read_unlock(), an srcu_read_lock() + call does not always have to match the next unpaired + srcu_read_unlock(). In fact, it is possible for two SRCU + read-side critical sections to overlap partially, as in the + following example (where s is an srcu_struct and idx1 and idx2 + are integer variables): + + idx1 = srcu_read_lock(&s); // Start of first RSCS + idx2 = srcu_read_lock(&s); // Start of second RSCS + srcu_read_unlock(&s, idx1); // End of first RSCS + srcu_read_unlock(&s, idx2); // End of second RSCS + + The matching is determined entirely by the domain pointer and + index value. By contrast, if the calls had been + rcu_read_lock() and rcu_read_unlock() then they would have + created two nested (fully overlapping) read-side critical + sections: an inner one and an outer one. + +3. The srcu_down_read() and srcu_up_read() primitives work + exactly like srcu_read_lock() and srcu_read_unlock(), except + that matching calls don't have to execute on the same CPU. + (The names are meant to be suggestive of operations on + semaphores.) Since the matching is determined by the domain + pointer and index value, these primitives make it possible for + an SRCU read-side critical section to start on one CPU and end + on another, so to speak. + +In order to account for these properties of SRCU, the LKMM models +srcu_read_lock() as a special type of load event (which is +appropriate, since it takes a memory location as argument and returns +a value, just as a load does) and srcu_read_unlock() as a special type +of store event (again appropriate, since it takes as arguments a +memory location and a value). These loads and stores are annotated as +belonging to the "srcu-lock" and "srcu-unlock" event classes +respectively. + +This approach allows the LKMM to tell whether two events are +associated with the same SRCU domain, simply by checking whether they +access the same memory location (i.e., they are linked by the loc +relation). It also gives a way to tell which unlock matches a +particular lock, by checking for the presence of a data dependency +from the load (srcu-lock) to the store (srcu-unlock). For example, +given the situation outlined earlier (with statement labels added): + + A: idx1 = srcu_read_lock(&s); + B: idx2 = srcu_read_lock(&s); + C: srcu_read_unlock(&s, idx1); + D: srcu_read_unlock(&s, idx2); + +the LKMM will treat A and B as loads from s yielding values saved in +idx1 and idx2 respectively. Similarly, it will treat C and D as +though they stored the values from idx1 and idx2 in s. The end result +is much as if we had written: + + A: idx1 = READ_ONCE(s); + B: idx2 = READ_ONCE(s); + C: WRITE_ONCE(s, idx1); + D: WRITE_ONCE(s, idx2); + +except for the presence of the special srcu-lock and srcu-unlock +annotations. You can see at once that we have A ->data C and +B ->data D. These dependencies tell the LKMM that C is the +srcu-unlock event matching srcu-lock event A, and D is the +srcu-unlock event matching srcu-lock event B. + +This approach is admittedly a hack, and it has the potential to lead +to problems. For example, in: + + idx1 = srcu_read_lock(&s); + srcu_read_unlock(&s, idx1); + idx2 = srcu_read_lock(&s); + srcu_read_unlock(&s, idx2); + +the LKMM will believe that idx2 must have the same value as idx1, +since it reads from the immediately preceding store of idx1 in s. +Fortunately this won't matter, assuming that litmus tests never do +anything with SRCU index values other than pass them to +srcu_read_unlock() or srcu_up_read() calls. + +However, sometimes it is necessary to store an index value in a +shared variable temporarily. In fact, this is the only way for +srcu_down_read() to pass the index it gets to an srcu_up_read() call +on a different CPU. In more detail, we might have soething like: + + struct srcu_struct s; + int x; + + P0() + { + int r0; + + A: r0 = srcu_down_read(&s); + B: WRITE_ONCE(x, r0); + } + + P1() + { + int r1; + + C: r1 = READ_ONCE(x); + D: srcu_up_read(&s, r1); + } + +Assuming that P1 executes after P0 and does read the index value +stored in x, we can write this (using brackets to represent event +annotations) as: + + A[srcu-lock] ->data B[once] ->rf C[once] ->data D[srcu-unlock]. + +The LKMM defines a carry-srcu-data relation to express this pattern; +it permits an arbitrarily long sequence of + + data ; rf + +pairs (that is, a data link followed by an rf link) to occur between +an srcu-lock event and the final data dependency leading to the +matching srcu-unlock event. carry-srcu-data is complicated by the +need to ensure that none of the intermediate store events in this +sequence are instances of srcu-unlock. This is necessary because in a +pattern like the one above: + + A: idx1 = srcu_read_lock(&s); + B: srcu_read_unlock(&s, idx1); + C: idx2 = srcu_read_lock(&s); + D: srcu_read_unlock(&s, idx2); + +the LKMM treats B as a store to the variable s and C as a load from +that variable, creating an undesirable rf link from B to C: + + A ->data B ->rf C ->data D. + +This would cause carry-srcu-data to mistakenly extend a data +dependency from A to D, giving the impression that D was the +srcu-unlock event matching A's srcu-lock. To avoid such problems, +carry-srcu-data does not accept sequences in which the ends of any of +the intermediate ->data links (B above) is an srcu-unlock event. LOCKING